ORIGIN OF LUMINESCENCE FROM POROUS SILICON DEDUCED BY SYNCHROTRON-LIGHT-INDUCED OPTICAL LUMINESCENCE

FOLLOWING reports of intense optical luminescence from porous silicon1 ,2, the opportunity for engineering optoelectronic devices using this material3,4 has attracted considerable attention. At present, however, the question of the origin of the luminescence has not been fully res olved5. The quantum-confinement model6-8 suggests that a quantum size effect gives optical transitions, and hence luminescence, in the visib le range - this idea gains support from the wavelength dependence of t he luminescence on porosity. An alternative model9,10 attributes the l uminescence to siloxene-like compounds11 formed on the silicon surface . A third model, which invokes hydrogenated amorphous silicon as a pos sible source12,13, seems to be contradicted by X-ray absorption fine s tructure (XAFS) studies14-16. Here we report optical luminescence in p orous silicon and siloxene induced by soft X-rays with energies near t he silicon K-edge (1,839 eV). Using the luminescence together with the total electron yield, we can obtain the XAFS spectra for the luminesc ent sites in both materials. Our results show that the luminescence fr om porous silicon does not derive from siloxene (either freshly prepar ed or annealed), and thus suggest that the quantum-confinement model s eems to provide the only viable explanation.